This invention relates to an air-fuel ratio detector which detects the air-fuel ratio of the presently operating state of an internal combustion engine of an automobile from exhaust gas components. More specifically, the present invention relates to an air-fuel ratio detector for detecting air-fuel ratios over a full operating range of the internal combustion engine of the automobile from a rich range to a lean range.
Air-fuel ratio detectors generally have a structure of a hollow pipe type oxygen sensor as disclosed, for example, in U.S. Pat. No. 4,210,510. Platinum electrodes are disposed on both sides of a zirconia solid electrolyte, and one of the platinum electrodes is exposed to the atmosphere while the other is exposed to the exhaust gas of the engine. This hollow pipe type oxygen sensor detects the air-fuel ratio from the point of sudden change of an electromotive force E (a point generally referred to as a "stoichiometric air-fuel ratio" and at which an air excess ratio .lambda.=1), the electromotive force E being generated stepwise by a difference of an oxygen partial pressure in the exhaust gas with the atmosphere being the reference. However, this sensor merely detects whether the air-fuel ratio is greater or smaller than the point .lambda.=1, but does not detect the state of the air-fuel ratios which change in a lean range where .lambda.&gt;1 and in a rich range where .lambda.&lt;1.
Therefore, an air-fuel ratio detector has been developed recently, as disclosed in U.S. Pat. No. 4,292,158, which measures the oxygen concentration in the exhaust gas and detects the air-fuel ratio from the oxygen concentration. In this air-fuel ratio detector, platinum electrodes are disposed on both surfaces of a zirconia solid electrolyte, and a cap having a space diffusion chamber, for example, is fitted to one of the surfaces. A diffusion hole through which the oxygen gas flows is disposed on the cap. This air-fuel ratio detector is exposed as a whole to the exhaust gas, and utilizes an oxygen pump phenomenon using the properties of the zirconia solid electrolyte. When a predetermined voltage is applied across both electrodes of a cell and the change of a pump current I.sub.P is measured, the quantity of this pump current I.sub.P is proportional to the oxygen concentration of the environment. Therefore, the air-fuel ratio .lambda. can be detected.
However, this sensor can not detect the stoichiometric air-fuel ratio, though it can detect the air-fuel ratio (.lambda.=1) in the lean range (.lambda.&gt;1).
Although the inventors of the present invention developed a detector by integrally combining two detection means, it was found that the two detection means interferred with each other and the detection accuracy was low for both the air-fuel ratio in the lean range and the stoicihiometric air-fuel ratio.